Spearfish Formation

Stratigraphic range: Permian–Triassic
Type| Geological formation
Underlies| Sundance Formation, Gypsum Spring Formation, Piper Formation
Overlies| Minnekahta Limestone
Thickness| 350–800 feet (110–240 m)[1]
Lithology
Primary| Siltstone, sandstone
Other| Shale
Location
Region| Black Hills and nearby states
Country| United States
Type section
Named for| Spearfish, South Dakota
Named by| Darton, 1899

The Spearfish Formation is a geologic formation, originally described from the Black Hills region of South Dakota, United States, but also recognised in North Dakota, Wyoming, Montana and Nebraska.[2] It is a heterogeneous red bed formation, commonly with siltstone and gypsum low in the formation and sandstone and shale higher up.[3] Other rock types include claystone, conglomerate, dolomite, and oil shale.[4] It is typically regarded as Permian–Triassic in age,[4] although its original description[5] included Jurassic rocks.[6]

The Spearfish Formation is interpreted as representing shallow marine to coastal terrestrial deposition, like the modern Persian Gulf.[7] Depositional environments may have included restricted marine bodies and ephemeral lakes (gypsum), hypersaline waters (limestone), hypersaline microbial mats (oil shale), and sabkhas (dolomite). The marine waters of the shallow continental sea retreated during the deposition of the formation, reflected in a change from dominantly nearshore marine to coastal terrestrial deposition over time.[4]

The Spearfish Formation is sparsely fossiliferous. So far, only stromatolites, casts of bivalves, and trace fossils have been found.[4]

## References[edit]

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1. ^ Driscoll, D.G.; Cater J.M.; Williamson J.E.; Putnam L.D. (2002). "Hydrology of the Black Hills Area, South Dakota" (PDF). USGS. pp. 11–15. Retrieved 29 December 2012.
2. ^ United States Geological Survey. "Geologic Unit: Spearfish". GEOLEX database. USGS. Retrieved 29 December 2012.
3. ^ Karner, F. R. and R. L. Patelke. 1989. General geology of the Black Hills and Bear Lodge Mountains. Pages 7-20 in Karner, F. R., editor. Volcanism and plutonism of Western North America: Devils Tower-Black Hills alkalic igneous rocks and general geology. Field trip T131 in Hanshaw, P. M., editor. Field trips for the 28th international geological congress. American Geophysical Union, Washington, DC.
4. ^ a b c d Sabel, J. M. 1984. Sedimentology and depositional history of the Permo-Triassic Spearfish Formation, southwestern Black Hills, South Dakota. Pages 295-307 in Goolsby, J. and D. Morton, editors. The Permian and Pennsylvanian geology of Wyoming. Wyoming Geological Association, Casper, WY. Guidebook 35.
5. ^ Darton, N. H. 1899. Jurassic formations of the Black Hills of South Dakota. Geological Society of America Bulletin 10:383-396.
6. ^ Richardson, G. B. 1903. The upper red beds of the Black Hills. Journal of Geology 11:365-393.
7. ^ Sabel, J. M. 1983. Sedimentology of Spearfish Formation. AAPG Bulletin 67(3):543-544.

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